It is well known to provide electronic article surveillance systems operating with "one-bit" EAS markers, i.e. markers whose presence can be detected by sensing equipment, but which otherwise provide no information. Such systems are widely used to prevent or deter unauthorized removal of items such as merchandise or library books from controlled premises.
It is desirable in some EAS applications to provide markers which are each capable of transmitting a unique multi-bit marker identification signal so that the presence of a particular item or individual associated with the marker can be detected. Systems using multi-bit markers for the purpose of controlling access to premises, or for keeping track of the locations of assets, have been proposed. In some cases, the proposed multi-bit markers are battery-powered, but providing a battery in the marker increases the cost of the system as well as the minimum size of the marker.
It has also been proposed to utilize active multi-bit markers that are powered by a field generated by detection equipment. For example, in the TIRIS system distributed by Texas Instruments, each marker includes a ferrite or wire coil antenna tuned to receive a power signal radiated by interrogation equipment at about 135 KHz. The marker also includes a storage capacitor which stores the received power signal and a memory which stores a unique multi-bit marker identification data word. The power signal also functions as an interrogation signal such that, when the storage capacitor is charged above a certain threshold, the marker automatically transmits a marker identification signal by radiating a frequency-shift keying data signal through the receiving antenna in accordance with the stored marker identification data.
It might be contemplated to operate field-powered active EAS markers at higher frequencies in order to increase the efficiency of power transfer to the marker so that the size of the antenna can be reduced and the range of operation increased. However, the use of a higher operating frequency also results in greater power consumption during transmission of the identification signal from the marker. As a result, in known toll road systems which operate at frequencies of several hundred megahertz to read tags provided on motor vehicles, either the tags include batteries or a narrowly focused power transmission beam is used. These tags also lack desirable features such as the ability to reprogram data stored in the tags.
Also, all existing multi-bit EAS systems utilize antenna structures that are too large for convenient attachment to many types of merchandise.